1. Field of the Invention
This invention relates to a method and the resulting article for providing a capacitance pressure transducer having a nearly zero temperature coefficient of capacitance under pressure. Such a transducer has particular application in the petroleum industry for accurately sensing down hole pressures within an oil or gas well.
2. Prior Art
Generally known to the prior art is a capacitance pressure transducer of the type having a pair of fused quartz cylinders which are concentrically aligned with and separated from one another so that a small gap is established therebetween. The capacitance of the transducer is dependent upon the size of the gap, such that changes in capacitance can be measured by an external circuit which is electrically connected to the capacitor electrodes. The electrodes are disposed in the gap and attached to opposing surfaces of the concentrically aligned cylinders so as to be responsive to pressure induced changes in the gap. Examples of the foregoing transducer configuration are available by referring to either of the following U.S. Pat. Nos.: 3,750,476, Aug. 7, 1973; 4,064,549, Dec. 20, 1977.
Ideally, each of the concentrically aligned cylinders should be at thermal equilibrium so as to avoid temperature gradient-related changes to the size of the gap. In practice, however, the temperature of the cylinders at the interior and exterior of the transducer assembly are frequently different. That is, both temperature and pressure are variable parmeters. Unlike pressure, the effects of temperature on a transducer assembly of the type hereinabove described are usually not instantaneously reflected. Therefore, prior to calibrating a pressure transducer of conventional construction and collecting pressure measurements therefrom, some time delay is initially introduced to permit the transducer cylinders to reach thermal equilibrium. Such delay results in an inefficient and time consuming pressure sensing scheme.
What is more, and as will be recognized by those skilled in the art, both temperature and pressure are variable parameters. Unlike pressure, the effects of temperature on a transducer assembly of the type hereinabove described are usually not instantaneously reflected. Therefore, prior to calibrating a pressure transducer of conventional construction and collecting pressure measurements therefrom, some time delay is initially introduced to permit the transducer cylinders to reach thermal equilibrium. Such delay results in an inefficient and time consuming pressure sensing scheme.
As will also be recognized by those skilled in the transducer art, the dimensions of the cylindrical transducer components are known to change with temperature. However, the significance of the temperature variation of the elastic constants of the transducer materials on the overall temperature coefficient of the transducer is not generally known to those skilled in the art. By way of particular example, a major contributing factor to the temperature coefficient of capacitance of a fused quartz pressure transducer is the temperature variation of the elastic constant known in the art as Young's modulus. For fused quartz, the temperature coefficient of Young's modulus is approximately 115 parts per million per degree C. The thermal coefficient of linear expansion of fused quartz is approximately 0.55 parts per million per degrees C. Thus, the temperature coefficient of capacitance of such a fused quartz transducer at any pressure other than zero is almost entirely a consequence of Young's modulus.
Capacitance pressure transducers have been fabricated in the past with materials characterized by a nearly zero thermal coefficient of linear expansion. One such suitable material is fused quartz. However, fused quartz characteristically becomes more rigid (i.e. Young's modulus increases) when heated. Because of this characteristic, fused quartz is not fully satisfactory for implementing a pressure transducer which will be subjected to either high or changing temperatures.
Accordingly, nothing is known which has suggested the fabrication of a capacitance pressure transducer from a particular transducer material which is characterized by certain parameter constants (i.e. the temperature coefficients of linear expansion and Young's modulus), such that the transducer assembly will advantageously have a much smaller temperature coefficient of capacitance under pressure than heretofor available from a conventional fused quartz transducer.